Cathode structure



July 14, 1931. F. s. M CULLOUGH 1,314,759

CATHODE STRUCTURE Filed Feb. 10, 1926 INVENTO R MIA/flay @w Patented July 14, 1931 PAZI'ENT OFFICE FREDERICK S. MGCULLOUGH, OF EDGEWOOD, PENNSYLVANIA CATHODE STRUCTURE Application filed February 10, 1926. Serial no. 87,312.

The present invention is for a cathode structure, and relates particularly to a cathode structure of the indirectly heated type for use in thermionic tubes and the like.

Cathodes of the indirectly heated type have been proposed for use in thermionic tubes of various types. I have heretofore proposed to provide a cathode structure in the form of a cylindrical metal shell fitted about a filling or spacing body of refractory material. Embedded or supported in the refractory material is a heating wire. Such a structure has the advantage that it may be heated, if desired,"from an alternating ourrent source.

The metal shell of such devices has preferably been made of nickel, because the magnetic properties of that metal serves as a shield to eliminate the effects of the alternating current field of the heater current on the output current of the tube. For the purpose of increasing the electron emissitivity of the shell, alkaline earth carbonates have been proposed as a coating for the shells.

Two disadvantages arise from such a construction. The nickel very readily acquires an oxide coating thereon which is detrimental to the tube and which tends to insulate the alkaline earth coating from the shell, thereby preventing a free current flow between the metal and the alkaline earth substance. The second disadvantage arises out of the fact that the shells are usually coated with the alkaline earth compound by immersion therein before the shell is applied to the refractory central rod or spacer, whereupon the rod or spacer, when forced into the shell, sc rapes some of the alkaline earth substance in the shell loose and this collects on one end of the refractory body while more of the interior coating, with its contained moisture, is trapped between the shell and the refractory. This not only renders thorough degassification difficult, but is a menace to the heating wire, any alkaline earth substance that contacts with the heater wire attacking the wire at the temperature and under the conditions found and existing during the pumping and subsequent operation of the tube.

The present invention proposes to eliminate these objections and provide acathode, and a method of producing the same, wherein the injurious oxidation of the shell is prevented and the alkaline earth substance applied in such manner that it is entirely out of contact with any of the heater structure.

The invention may be readily understood by reference to the accompanying drawings which illustrate the invention more or less diagrammatically, and to the details of which my invention is not confined.

In the drawings:

Fi ure 1 is a longitudinal section through a cat ode structure embodying my invention.

Figure 2 represents a strip of metal from which the cathode shells may be made.

Figure 3 a is a perspective view of the cathode structure.

In the drawings, 5 is a cylindrical metal shell within which is a heater wire 6. The heater wire is mounted in the shell by suitable spacing means, such as the mass or rod of refractory material or porcelain 7 in which the wire is embedded. I have shown one end of the heater wire connected to the cathode at 8, but this may be changed as desired, as in some cathodes theheater wire need not connect to the shell.

The shell is preferably formed of nickel, as hereinbefore explained, and in order to protect it against undesirable oxidation before and during the pumping of the tube in which the cathode is mounted, the metal is plated with a thin coating of a metal less like ly to form an oxide coating over itself than nickel. This coating is preferably silver, as silver does not so readily acquire an oxide skin or film as does the nickel. The emitting coating of barium or strontium carbonate or other substance is applied to the silver plated shell. This coating is designated 9.

In the assembly of the structure, the heater wire and porcelain are assembled in the usual manner. The nickel, which may be in thin strips 10, as shown in Fig. 2, may be plated with silver before or after annealing, the annealing being effected to. render the nickel pliable.

.Pieces of the annealed and plated nickel are then bent into the form of the cathode shell and the heater structure is fitted therein. After the structure has been assembled, a coating, such-as barium and strontium carbonate in water, is spread over the shell, preferably with a brush, care being taken to prevent this coating from running over the edges of the shell onto the heater structure. In order to insure perfect isolation of the coating material, a narrow, uncoatedumargin 11 is left at each end of the shell.

From the foregoing, it will be evident that i J nickel or any other suitable material may be used for the shell, and that by the application of the metal coating thereto it can be protected against the formation of a semi-insulating oxide film, the metallic coat on the plate serving to more intimately unite the shell with the emitting coating. This plating of the metal not only-protectait prior to its incorporation into the tube. but also protects it during the pumping of the tube and any operation subsequent thereto.

By painting the alka ine coating on last, it is assured that the alkaline materials may be applied only where desired and only where the material can be thoroughly dried and degassified. In place of barium and strontium carbonates, other well known emission materials may be used.

What is claimed is:

1. In a cathode structure, a cylindrical shell of nickel open at each end thereof, a refractory mass within the shell, a heating wire passing through the refractory mass, a protective coating of metal over the shell, such protective coating being more resistant to oxidation than the nickel, and a coating of emitting substance on the outside of the shell over the protective coating.

'2. A cathode unit adapted to be mounted and enclosed-in a thermionic tube, comprising a thin cylindrical shell of nickel open at each end and rendered proof against oxidation, a refractory mass within the cylindrical shell, a heater wire supported by the mass out of contact with the shell, and an emitting substance on the outer surfaceof-the shell.

In testimony whereof I have hereunto set my hand.

FREDERICK S. MGCULLOUGH. 

